Composite oxygen transport membranes have been proposed for a variety of uses that involve the production of essentially pure oxygen by separation of oxygen from an oxygen containing feed through oxygen transport through such membrane. For example, such membranes can be used in combustion devices to support oxy-fuel combustion or in reactors for partial oxidation reactions involving the production of a synthesis gas or generation of heat to support endothermic chemical reactions.
In such applications, the composite oxygen transport membranes contain a dense layer of a mixed conducting material that allows transport of both oxygen ions and electrons at elevated temperatures. The dense layer is formed on a ceramic substrate that functions as a porous support. The dense layer can be composed of a mixed conductor or two phases of materials, an ionic phase to conduct the oxygen ions and an electronic phase to conduct the electrons. Typical mixed conductors are formed from doped perovskite structured materials. In case of a mixture of materials, the ionic conductor can be yttrium or scandium stabilized zirconia, and the electronic conductor can be a perovskite structured material that will transport electrons or can be a metal or metal alloy or a mixture of the perovskite type material and metal or metal alloy. Some known membranes also have additional layers such as a porous surface exchange layer located on the feed side of the dense layer to enhance reduction of the oxygen into oxygen ions, and an intermediate porous layer on the opposing side of the dense layer. Such a composite membrane is illustrated in U.S. Pat. No. 7,556,676 that utilizes two phase materials for the dense layer, the porous surface exchange layer and the intermediate porous layer. These layers are supported on a porous support that can be formed of zirconia.
In order to minimize the resistance of the membrane to the ionic transport, such membranes are made as thin as practical and are supported on a porous support. Since the resistance to oxygen transport is dependent on the thickness of the membrane, the dense layer is made as thin as possible and therefore must be supported. Another limiting factor to the performance of an oxygen transport membrane concerns the supporting layers on either side of the dense layer; these supporting layers may or may not be active for oxygen ion or electron conducting. These layers themselves can consist of a network of interconnected pores that can limit diffusion of the oxygen, or fuel or other substance through the membrane to facilitate oxygen transport and enhance oxygen flux across the membrane. Therefore, such support layers are typically fabricated with a graded porosity in which the pore size decreases in a direction taken towards the dense layer or are made highly porous throughout. The high porosity, however, tends to weaken such a structure. The resulting composite oxygen transport membrane can be fabricated as a planar element or as a tubular element in which the dense layer is situated either on the inside surface or the outside surface of the planar element or tube.
The composite oxygen transport membranes function by transporting oxygen ions through a material that is capable of conducting oxygen ions and electrons at elevated temperatures. An oxygen containing stream flows on one side, retentate side of the membrane, at least a portion of which contacts the membrane surface. Oxygen in the contacting oxygen containing stream ionizes on the membrane surface and the resultant oxygen ions are driven through the mixed conducting material and emerge on the opposite side thereof to recombine into elemental oxygen. In the recombination, electrons are liberated and are transported back through the membrane to the retentate side to begin the ionization cycle. The permeated oxygen reacts with a fuel flowing on the permeate side of the membrane. The combustion reactions produce products such as synthesis gases by means of partial oxidation of the fuel. It is to be noted that the combustion reactions by combusting at least some of the permeated oxygen produce a difference in oxygen partial pressure across the membrane that can serve as a driving potential for oxygen transport across the membrane. The combustion reactions also produce heat that is used to raise the temperature of the membrane to an operational temperature at which the oxygen transport can occur. Heat in excess of that required to maintain the membrane at a desired operational temperature can be utilized to supply heat to an industrial process that requires heating. In syngas production applications the fuel stream introduced on the permeate side typically contains combustible species such as hydrogen, carbon monoxide, methane. In some instances other hydrocarbons may also be present in the fuel stream. Unreacted combustible gas leaves with the effluent on the permeate side.
Use of oxidation catalysts have been proposed to enhance syngas production. The oxidation catalysts can be incorporated within mixed conducting layer through which oxygen transport occurs or the oxidation catalysts can be disposed within the membrane as a contiguous layer to the mixed conducting layer. For example, U.S. Pat. No. 5,569,633 discloses surface catalyzed multi-layer ceramic membranes having a dense mixed conducting multicomponent metallic oxide layer with a first surface contiguous to a porous support surface and a second surface coated with catalyst material to enhance oxygen flux by catalyzing reactions with oxygen separated from an oxygen containing feed gas. Unexpected benefit of higher oxygen flux was observed upon coating the membrane surface in contact with the oxygen containing feed gas with catalytic material. However, such solutions utilizing oxidation catalysts initially accelerate the oxygen flux but the performance deteriorates due to the intense redox cycles experienced by the oxidation catalyst material, resulting in membrane cracks and functional layer delamination. U.S. Pat. No. 8,323,463 discussed impregnating the intermediate porous layer including a layer of porous support contiguous to the intermediate porous layer with catalysts such as gadolinium doped ceria to promote oxidation of a combustible substance, and thus increase oxygen flux. U.S. Pat. No. 4,791,079 advocated the integration of impervious mixed conducting ceramic layer with a porous catalyst for hydrocarbon oxidation or dehydrogenation. Lithium or sodium promoted manganese complexes were suggested as preferred catalysts. U.S. Patent Publication No. 2006/0127656 applied a porous catalytic layer adjacent to the mixed conducting dense layer for catalytic partial oxidation of hydrocarbons.
Use of reforming catalysts has also been proposed to enhance syngas production by converting the unreacted hydrocarbon present on the permeate side. The reforming catalyst can be positioned proximate to the membrane permeate side as distinct catalyst elements separate from the membrane. Examples of such distinct catalyst elements include structured catalyst inserts in the form of pellets, foils, mesh structures, monoliths and the like. However, such solutions add pressure drop and complexity. The need continues to exist to advantageously deploy reforming catalyst to get higher synthesis gas yield, convert more of the methane in feed stream to synthesis gas by reforming reactions, and manage heat released from combustion reactions within the membrane to support endothermic reforming reactions. The reforming catalyst should not adversely affect oxygen flux, neither introduce contaminants into the mixed conducting oxygen transport layers nor cause structural and/or functional degradation.
As will be discussed the present invention provides a dual function composite oxygen transport membrane and a method of manufacturing the article itself. More specifically, the invention relates to a dual function composite membrane that separates oxygen as well as catalyzes reforming reactions, wherein said dual function composite membrane comprises a ceramic substrate with a mixed conducting dense layer on one side of the substrate for oxygen transport, and a catalyst layer on the opposing side of the substrate for catalyzing endothermic reforming reactions. The membrane is produced by depositing the mixed conducting dense layer and the catalyst layer on the opposing sides of the substrate in separate steps. The catalyst layer is formed using catalyst material selected to promote endothermic reforming reactions thereby to convert hydrocarbon in the permeate side reaction mixture into syngas.